Recording wattmeter



y 1, 1953 A E. c. GREGG, JR 2,646,547

RECORDING WATTMETER Filed July 6, 1950 F IG.I.

D R RECORDER I RECORDER RE- GORDER INVENTOR EARLE G. GREGG, JR.

ATTORNEYS Patented July 21, 1953 UNITED STATES PATENT OFFICE RECORDINGWATTMETER Earle. C. Gregg, Jr., East Cleveland, 'Ohio, as-

signor to. the. United States of America as rcp resented by the.Secretary of; theNavy Application July-6, 1950, Serial'No.172,259

of power input into the devices,- based on impedance bridge tests atvery lowlevels, could at best only give approximate results. The need,consequently, was great to have a means for measuring actual power inputdirectly, and to take care 01 these needs the present invention has beendeveloped for the measurement of power over the large frequency rangesuch as may'be used in underwater acoustic measurements.--

Wattmeters, employing- V resistors, thermocouples, and ameter forreadingpower as a' function of the currents generated by thethermocouples have been designed. However, while the thermocouplewattmeter is direct reading and fairly accurate, it still possesses anumber of disadvantages, the most important of which are: (1) the factthat thermahlags necessitate a delay in reading, which makes necessarypoint by point measurements; (2). the-possibility of. destruction.

of the thermocouples, 3 resulting from. sudden.

h s n lo d da ce. a the limite impedance range for any one powerscale.These difficulties are largely overcome in the. design of,

the wattmeter of the present invention.

One of the principalobjects of the present invention is to provide meansfor measuring power inv a load. circuit. over a relatively wide range offrequencies. Another object is. to pro.- vide means whereby power,voltage, or current in a load circuit may be measured as desired.Another objectis to provide means which. is not delayed by thermal lagwhen it measures-power;

claims, reference is now made to the following description taken. inconnectionwithrthe accoms.

panying drawings in. which; s i Fig.v 1.1isa schematiecireuit diagram.of one embodiment of the.- invention; when. used; in an.

unbalanced circuit; and,

Fig. 2 is a schematic circuit diagram of; one.

'6Claims. (Cl. 324-142)) side, of impedance 23.

embodiment of the invention when used in a balanced circuit. v

Referring now to the drawings, wherein like reference charactersdesignate like or correspond ing parts throughout the several views,thereis shown in Fig. l a source I I of alternating current power whichis connected to terminals A and B- so as to furnish power'to anunbalanced load cir-" cuit [2 by means of power lines I3 and I4.Connested betweenv lines [3 and. l lrsov as to be parallel to loadcircuit 12, across source I I, is a substantially Y-sha'ped circuit for.measuring the power supplied. to the load. This Y-shapecl cir-' cuitcomprises one embodiment. of the present invention.

Connected to line I3 at point l5, and forming the leg portion of theY"-circuit, is an impedance" device I6 which joins, at common point l1,one terminal of each of two other impedance devices [8. It is to benoted that the impedances I8 are equal. in value and" each one forms; anarm for the Y'-circuit. Devices i8 join'p'ower' line I4 at points 2|and22 respectively,while'between junc tion points 2! and 22, andconnected'in series with line 14,. is another-impedance device 23.Directly inafter...

,.When th invention is employed to measure power in abalanced circuitthe arrangement is shown in Fig.2 wherein the power source! I isconnected. to the primary 25v of a transformer having two secondarywindings 26 and, 21. which, are arranged,v to supply power to the loadcircuit- One. terminal of. secondary winding 26 is i 2 L joined. to.line I l 3.. while: its other terminal connects with. one side ofimpedance 2.3 which. is placed-- in series between the two secondarywindings.

Secondary wind-ing, 2.7joins line. I 4: and the other Also-connectedacross the terminals ance; 23 is a double-pole double-throwswitch Z8which is cross connected" so that it functions as areversing switch,Each; of the arms. of. switch 2:8. is tied toan: impedance WA, whilebetween the terminals of theseimpedances, there is placed impedance:'i8,.and' it can beclearlyzseen from the notations on Fig. 2 that: the:valuesof impedances IBA are selected as'exactly one-half that of im---pedancel 8.. On either side of impedance l 8, and connected between *itand the lines l3 and M.

respectively, is. an impedance. [6A which, it will. be noted, is takento::be.ione-ha1f the'value of of imped- 3 impedance 1 6 of themodification shown in Fig. l. The impedances of the present inventionbear a definite predetermined relation relative to each other, and it isobvious from Fig. 2 that while the relation between these values ismaintained, at the same time, a balanced circuit is obtained by equallydividing some of these impedances and putting half on each side of thecircuit.

Connected across impedance 18 (Fig. 2) is a recorder 24 for recordingeither the sum or difference of the currents flowing through l8, depnding upon the position of switch 28. It may be mentioned at this pointthat should it be desirable to have the wattmeter of the presentinvention used as a portable instrument, that the recorder 24 may bereplaced by an amplifier and =ei cos 0: ower 4mm 4mn p where 0 is thephase angle between the current and voltage. The S and D signals arerecorded on the intermediate frequency recording systems in the usualmanner. In the circuit of the invention, the measurement of total poweris correct only for sinusoidal wave forms, and as previously mentionedfor portable use a separate amplifier and meter may be used in place ofthe recorder.

Figs. 1 and 2 show the basic circuits of the recording wattmeter used inobtaining the S and D terms for unbalanced and balanced circuits. In theactual circuit for the unbalanced case (Fig. l) a coil and a pad (smalladjustable condenser) are used for either of the impedances H3 in orderto obtain the S or D signal; which is measured across the impedance l8as clearly illustrated in Fig. 1. Should it be desirous to measure onlythe voltage and current components they may be obtained by shortcircuiting impedance 23 and dis connecting impedance [6, respectively.By short circuiting 23, there remains only impedances l6 and i8,connected across the line, in parallel with load I2, so as to give ameasure of the voltage across the load. When impedance i6 isdisconnected, then the reading obtained is the current flowing through23 and this is proportional to the current through load l2 sinceimpedance 23 functions as a shunt in line 14 to tap off a proportionatepart of the load current.

' In the balanced condition (Fig. 2), the only new factor that need beconsidered is the stray capacity between the two high potentialterminals of the driving coil. The effect of the stray capacity may ingeneral be neglected for the usual load impedances encountered. Theimpedance of the secondary windings will have no elfect on themeasurements of power.

It can also be shown that if impedance 23 is center-tapped to ground,the meter will measure the power delivered to both the load impedanceand the impedance to ground. However, if no ground connection is made(as shown in Fig. 2), the meter will read the load dissipationindependent of the degree of unbalance.

The S or D signal, depending upon the direction in which the reversingswitch 28 is thrown, will appear across impedance [8 (consisting of apad plus coil). Voltage and current components are obtained in the samemanner as in the unbalanced case of Fig. 1, namely, by shortingimpedance 2E! and disconnecting impedances 5A, respectively.

In operation, when the power source is furnishing power to load l2, andthe device is being used 'to measure this power, then current flow inthe various branches of the circuit is in the directions shown in Fig.l, for example. Kirchoifs law will apply. Assuming the total current inthe system to be i, and flowing to the right (Fig. l), and neglectingthe effect of impedance R2 for the moment, then the current through H5will be downward and equal to 22'1, with equal current components i1,flowing through each of the impedances l8. However, due to the potentialdrops around the circuit and across impedance 23, there is anothercurrent, i2, flowing counterclockwise around the arms of the Y -shapedportion of the circuit. It is obvious then that the signal meas uredacross the right-hand impedance it will be equal to D, or the difierencebetween the two currents i1, and i2, while the signal across thelefthand impedance will be equal to S, or the sum of the currents i1,and i2. Likewise, in Fig. 2, it can be shown that a similar S or Dsignal will appear across impedance l8 depending upon the position ofswitch 28. Therefore, once the sum and difference signals have beengotten the power can be obtained.

In some embodiments of the wattmeter, the system can be changed from thebalanced condi tion to an unbalanced condition by a switch on the frontpanel. In actual operations the wattmeter is frequently employed tomeasure power in underwater acoustical experiments and four records areobtained on the chart paper; via, S, D, E and I. Since the levels arerecorded in decibels versus 1O watts, the subtraction of a properconstant, K, from the recorded S level will give in decibels versus onewatt. The same procedure and the same constant are used to obtain the Dreading, and thus when these two readings are converted to watts, thedifierence between them will be the power delivered to the load. Similarprocedures are used to convert the voltage signal to decibels versus onevolt and the current signal to decibels versus one ampere.

It can be shown that the errors in reading the wattmeter are a functionof the ratio me/m', or the ratio of the voltage signal to the currentsignal, and so three impedance ranges have been incorporated to keepthese errors to a minimum. If the records of current and voltage differby more than 6 to 8 decibels in a particular region, selection of theimpedance range which brings the two signals closer together, and henceimproves the accuracy of the power reading, is made by means of a switchon the wattmeter.

The only element determining the frequency characteristic of thewattmeter is the insertion loss of the coil replacing the impedance i8,and this Will appear as a variation, with frequency, of the conversionconstants. The variation in the conversion constants over the frequencyrange of 50 cycles to kc. is less than 0.4 decibel.

The wattmeter of the present invention may be used over a power range of0.001 watt to 1500 watts with an accuracy on the order of one to fourpercent for phase angles up to 85 degrees. The total impedance range isfrom 15 to 800 ohms.

In the light of the above disclosure, it can be seen that the wattmeterof the present invention represents a definite improvement over theprior art devices and offers a simple and reliable means for measuringpower in both balanced and unbalanced circuits. It may be used to readpower, voltage or current, as desired, and contains no thermal elementswhich may be easily damaged and which inserts a thermal lag'into thereadings.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention as setforth in the appended claims.

I What is claimed is:

1. In combination with a power circuit, a transformer having a pair ofsecondary windings connected to a load, a first impedance in seriesconnection between the secondary windings, a second impedance and twofirst small impedances in series relation connected across the seriesconnected secondary windings, said second impedance being positionedbetween said small impedances, switching means connecting the first andsecond impedances in parallel, and a second small impedance in each ofthe leads connecting the switching means and the second impedance.

2. The combination of claim 1 wherein each of the said second smallimpedances is one-half the value of the second impedance.

3. In combination with a power circuit, a transformer having a pair ofsecondary windings with a first large impedance in series therebetween,a load across the secondary windings, a

. 6 second large impedance positioned between two first small impedancesand in series therewith to form a path parallel to the load, a secondsmall impedance connected to each terminal of the second largeimpedance, and a reversing switch between the first large impedance andthe second small impedances whereby a sum or difference signalproportional to the current and voltage of the load appears across thesecond nected between said substantially equal impedances, a pair ofleads, reversing switching means connected to the ends of said firstimpedance and to said pair of leads, said leads being connected acrosssaid second impedance, and a further impedance in each of said leads,said further impedances being substantially equal.

6. A measuring circuit as defined in claim 5 but further characterizedby each of said further impedances being substantially one-half 01' saidsecond impedance.

. EARLE C". GREGG, JR.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,059,594 Massa Nov. 3, 1936 FOREIGN PATENTS Number CountryDate 328,035 Great Britain Apr. 14, 1930 511,367 Great Britain Aug. 17,1939 708,949 Germany Aug. 1, 1941

